The present disclosure relates apparatus and methods of non-contact, optical, characterization of semiconductors and semiconductor devices. More particularly, the present disclosure relates to the optical characterization of solar cells.
The modern solar cell industry has moved toward GW-scale production, with quality control becoming a critical factor [15]. Conventionally, the I-V characteristics are obtained by attaching a resistive load or a power source to the irradiated solar cell to measure and evaluate its efficiency. This practice cannot meet the needs of mass production.
Non-destructive and non-contacting methods for optoelectronic diagnostics of solar cells at all stages of the fabrication process are in strong demand. Several such methodologies have been developed for analyzing the excess charge carrier lifetime of Si wafers in a short time, including Carrier Density Imaging (CDI) [16]. Microwave photoconductance decay (MW-PCD) is a “golden standard” method for imaging lifetimes, including short recombination lifetimes, but it is much more time-consuming [17].
Imaging techniques based on quasi-steady-state (DC) electroluminescence (EL) and photoluminescence (PL) are widely used for qualitative and quantitative characterization of silicon solar cells [1-3]. PL imaging (PLI) is a fast non-destructive and non-contacting camera based diagnostic method which has been used for detecting electronic and other defects associated with crystal imperfections and handling of solar cells [3, 18-20]. However, DC PL cannot monitor the optoelectronic carrier kinetics of surface and near-subsurface regions due to its depth-integrated character through the signal dependence on the DC carrier diffusion length [3].
Harmonic and square-wave modulated photoluminescence [4a,b] is generally a non-linear process of electron-hole band-to-band recombination at high photoexcitation densities, with signals quadratic in the excess photocarrier density. With Si substrates, modulated photoluminescence requires very high frequencies (100 kHz-10 MHz) to monitor fast (˜2.9 μs) interband-gap decay times [4a], whereas camera-based dynamic photoluminescence imaging of solar cells is attainable at very low frequencies (˜25 Hz) [4b].
PL is a radiative emission process which can be interfered with by broad spectral contributions, such as overlapping thermal emissions due to lattice absorption, non-radiative recombination and thermal photon emission (Planck radiation). Laser-induced infrared photocarrier radiometry (PCR) [5, 27] is a quantitative dynamic near-infrared (NIR) modulated PL, spectrally-gated to filter out the thermal infrared component of the radiative emission spectrum from de-exciting free photocarriers, which is governed by the Law of Detailed Balance on which the non-equilibrium kinetics of optoelectronic device operation is based [21].
The infrared spectral complement of PCR concerns Planck (blackbody) thermal emissions due to nonradiative carrier de-excitations and can be detected using photothermal radiometry (PTR), a modulated thermal-wave generation and detection method [22]. The imaging equivalent of PTR is lock-in thermography (LIT) which has also been used to investigate local power losses in solar cells [23-25]. PCR has proven to be an effective non-contact methodology for the measurement of transport properties in semiconductors [5,26].
Lock-in carrierography (LIC), the dynamic imaging extension of PCR, was recently introduced using a spread superband-gap laser beam and a near-infrared (NIR) InGaAs camera [6]. However, in implementing this technique, only low modulation frequencies (≦10 Hz) could be used in order to maximize image signal-to-noise ratio (SNR) through oversampling.
There remains a need for improved spatial (radial and axial) resolution characteristic of optoelectronic defects at frequencies much higher than those achievable by today's state-of-the-art InGaAs camera capabilities.